1. Field of the Invention
The present invention relates to a method for producing a devolatilized polyolefin, the method being capable of preventing water fed into a twin screw extruder from flowing backwards during water injection foaming devolatilizatin of a polyolefin and hardly allowing cavitation to occur in a water injection pipe, and to a molded polyolefin article superior in low fogging property.
2. Related Art
Heretofore, plastics have been used for automotive interior materials because weight reduction or diversified design can be achieved thereby. Especially, polyolefin-based resin compositions have been used because of their less inclusion of plasticizers, which will cause fogging, and in view of their influence to the environment caused after their disposal.
JP 10-36578 A, has disclosed a resin composition for an automotive interior material, the composition comprising an olefin-based resin, a thermoplastic elastomer, and an additive and having been prepared by kneading these materials for 30 seconds by using a twin screw extruder under heating at 240 to 250° C.; JP 5-116205 A has disclosed a method for producing a polyolefin with low odor by adding water to a polymer at a hopper, and performing devolatilization by using an extruder with a vent; and JP 7-88927 A and JP 10-249913 A each have disclosed a method for removing volatile components by water injection foaming devolatilization performed within a twin screw extruder with a vent. It has been disclosed that thanks to the provision of a ring in a dispersion zone, water is successfully distributed uniformly into resin by filling molten resin into that zone and injecting water thereinto with pressure, and then volatile components contained in the resin are removed by performing foaming devolatilization at a devolatilization zone provided downstream from that zone.
However, the resin composition disclosed in JP 10-36578 A may suffer from degradation of resin and deterioration in low fogging property due to the presence of the degraded resin because the resin composition is kneaded at high temperature; the method disclosed in JP 5-116205 A may result in the occurrence of feeding failure because when water is injected through a hopper together with resin in a powdery or pelletized form, the resin and the water may not be mixed successfully in a kneading zone, causing backflow of the water; and in the method disclosed in JP 7-88927 A and JP 10-249913 A, in the case that the cylinder of the extruder is provided with a water injection port within a kneading zone to be filled up with resin and equipped with a water injection pump via a water injection pipe, when water is pumped into the cylinder through the water injection port via the water injection pipe, the water may boil in the water injection pipe to cause cavitation in the pipe, so that it may become difficult to inject water.
Under such situations, the problem to be solved by the present invention is to provide a method for producing a devolatilized polyolefin, the method being capable of preventing water fed into a twin screw extruder from flowing backwards within the extruder during water injection foaming devolatilizatin of a polyolefin and hardly allowing cavitation to occur in a water injection pipe, and provide a molded polyolefin article superior in low fogging property.